Breeding of leafy vegetables like lettuce aims at the production of commercial varieties optimally adapted to produce marketable products. Many characteristics need to be taken into account during selection which relate to both input and output traits. One of the most important traits in this respect relates to post-harvest quality, in particular to shelf life. The avoidance of physiological disorders and more in particular Russet Spotting and Yellowing are important elements that can extend the shelf life of a lettuce crop or parts thereof.
Ethylene is a plant hormone generally known to stimulate physiological processes related to senescence. In lettuce this stimulation becomes apparent through the formation of symptoms such as Russet Spotting and Yellowing.
The Russet Spotting disorder is characterised by the appearance of brown spots alongside the midrib of the leaves whereas Yellowing is the general bleaching of leaves which occurs during senescence as a consequence of chlorophyll breakdown.
Although mature heads of lettuce are known to produce only minute amounts of ethylene, the plants are highly sensitive towards this plant hormone. Therefore, physiological disorders associated with ethylene sensitivity which reduce the post-harvest quality of lettuce are mainly caused by external sources of ethylene. Exposure to such external sources can occur during harvesting, processing and storage of the produce.
For example, when lettuce is transported or stored in the vicinity of ethylene producing fruits such as apples, pears or peaches severe deterioration may occur. Furthermore, when lettuce is processed and used in packaged fresh-cut mixtures there may be limitations with respect to the ingredients which can be used due to ethylene release by one or more of the ingredients.
Russet Spotting is a physiological disorder which is manifested by the appearance of numerous brown spots along the midrib of the leaf. The browning symptoms can spread all over the leaf during the progressive stages of the disorder. Russet Spotting is known to occur especially when mature lettuce heads are stored at lower temperatures (5° C.) in the presence of low concentrations (ppm levels) of ethylene.
Symptom formation can be antagonised by applying the plant hormone auxin or calcium. Furthermore, modified atmospheres containing low oxygen levels reduce the speed at which symptoms develop.
At the biochemical level Russet Spotting appears to develop as a consequence of a local stimulation of lignin biosynthesis, which causes lignification and cell wall thickening around the area of the leaf where the visual symptoms will appear.
The brown discolouration is caused by the stimulation of phenolic metabolism. The enzyme phenylalanine ammonia lyase (PAL), which has been shown to be induced by ethylene, catalyses the first committed step of the phenylpropanoid pathway. Phenolic compounds which are formed mainly include caffeic acid derivatives as well as a number of flavonoids such as (+)catechin and (−)epicatechin. Subsequent oxidation of these compounds by polyphenol oxidase (PPO) leads to the brown discolouration typically observed in Russet Spotting. Finally, the symptoms may become more severe due to collapsing of tissue and cell death.
Senescence is a naturally occurring, developmental process at the end of a life cycle of a plant or plant organ during which metabolism is reprogrammed in order to remobilize resources into reproductive structures like seeds. Although senescence is a developmental process caused by endogenous factors like physiological age, there are many exogenous factors which can modulate senescence.
Yellowing of leaves, the most visible symptom of senescence, is a consequence of chlorophyll breakdown during a relatively late stage of senescence, which can be enhanced by ethylene once a leaf is receptive. Well-known other stimulating factors of senescence are wounding, darkness and nutrient deficiency. Although ethylene is the most important plant hormone known to stimulate senescence, other hormones like jasmonate may also contribute to this process.
From the moment of harvest of the lettuce crop until the moment of consumption, the produce can be exposed to the different exogenous factors contributing to senescence. These can be wounding during harvesting and processing, darkness and nutrient deficiency during storage and ethylene during processing and storage. These factors strongly stimulate the post-harvest disorders which can become apparent as Russet Spotting and Yellowing. Although these effects are largely cosmetic the product becomes much less attractive and thereby unmarketable.
In order to counter the deterioration effects, many post-harvest measures can be taken which reduce these effects. For example, one can store the harvested lettuce at low temperatures to retard senescence. Although this may reduce the rate of Yellowing, Russet Spotting may be enhanced. In addition, logistic measures may be implemented that reduce the transportation time required from the field to the consumer or that prevent the lettuce from being stored in the vicinity of an ethylene source. Furthermore, chemical treatments may be applied, which prevent the post-harvest deterioration, although food safety and consumer acceptance obviously become an issue.
Many of the post-harvest measures are successful to some extent but there is certainly room for improvement. Moreover, costs involved may be substantial, which is another reason to explore alternatives that reduce the need to apply post-harvest treatments. Preferably, a genetic solution is found which reduces or eliminates the need to take the expensive, preventive measures that are currently used to maintain the post-harvest quality at a high level.
It is the object of the present invention to provide a screening method to identify ethylene-insensitive plants. It is a further object of the invention to provide plants that are obtainable by the method.